(1) As shown in FIG. 14, a heat insulating wall 13 of a body 15 in a refrigeration vehicle or cold reservation vehicle 10 typically has the structures disclosed in the following:                a. A sandwich panel 13A formed by adhering a slab 131, made of expanded urethane foam or expanded styrene foam or the like (already expanded and formed into a plate-shape), to inner and outer panels 13a, 13b, made of a plate-shaped flattened aluminum material, FRP, or steel, by use of an adhesive 13c; or        b. A panel 13B shown in FIG. 15, where an independent expanding urethane resin is injected and expanded between inner and outer panels 13a, 13b to form a portion 133, and the inner and outer panels are bonded with the self-adhering force of urethane.        
Normally, the heat insulating walls 13 of the body 15 are assembled into a box-shape, with the six walls, the ceiling, the floor, the two side walls, the front wall and the rear door, being formed with the above mentioned panels (13A or 13B) having either the structure mentioned in a. or in b.
(2) One desire is that the heat-insulating container of vehicles have increased capacity within the container (body).
In order to increase the capacity of the container easily, the width and the height of the container should be enlarged. However, the outer frame size of the container is limited by regulations.
Therefore, in order to increase the inner size of the container without increasing the outer frame size of the container, there is a need to decrease the thickness of the wall (insulating member).
Of course, in order to decrease the thickness of the wall without deteriorating the insulation performance, it is indispensable to improve the insulating characteristics of the wall.
One way to improve the heat insulation performance of the wall is to apply to the wall a vacuum insulation panel having a lower heat transfer rate than the conventionally used expanded plastic foam material.
The heat conductivity of heat insulating materials is shown in Table 1.
TABLE 1Heat conductivitySample Materials[kcal/m · h · ° C.]Polyurethane foam0.020-0.022Polystyrene foam0.024-0.034Vacuum insulation panel (filled withapproximately 0.007continuous expanded urethane foam,degree of vacuum: 10−2 Torr)Continuous expanded urethane foamapproximately 0.045unit (without aluminum laminated film,atmospheric pressure)
Based on the heat transfer rate values shown in the above table, when calculating the necessary thickness of the heat insulating material when replacing the conventional polystyrene foam with a vacuum insulation material, the same heat insulating performance is obtained with the thickness of the wall reduced to approximately ¼.
As explained, by utilizing a vacuum insulation member, the thickness of the insulating wall may be reduced without deteriorating the insulating performance. Therefore, the capacity within the container may be increased, and the weight of the container may be reduced.
Even further, when the vacuum insulation material is employed in the wall without changing the thickness of the wall, the heat insulating performance may be quadrupled, and the fuel consumption rate will be improved.
For the above reasons, a wall including vacuum insulation material is already applied to portions of domestic (home) refrigerators. The wall structure applied to a domestic refrigerator utilizing vacuum insulation panels is explained with reference to FIG. 16.
In a domestic refrigerator 20, vacuum insulation panels 25 are incorporated in the outer walls 24 of a cooling chamber 21, a refrigerating chamber 22 and a vegetable chamber 23, the interior temperature of which must be maintained at about 4-5° C. or at about −18° C.
The vacuum insulation panel 25 is formed by placing a continuous expanded urethane foam 25a inside a bag 25b made of aluminum laminated film, and under vacuum condition the bag is airtightly sealed by a seal portion 25c. The outer panel 24 of the refrigerator is formed by placing the vacuum insulation panel 25 between a flat steel outer plate 24a and a molded inner plate 24b made of three-dimensionally deformed ABS resin and the like formed by vacuum molding. The vacuum insulation panel 25 is fixed to the outer panel 24a by a hot-melt adhesive or a double-coated tape. An independent expanding urethane foam 24c is injected and expanded in the space formed between the vacuum insulation panel 25, the inner panel 24b and the outer panel 24a. 
In the wall structure, the bond between the outer panel 24a, the inner panel 24b, the aluminum laminated film 25b and the independent expanding urethane foam 24c is stronger than the self-adhering power of the urethane foam. Therefore, in a domestic refrigerator, there is no need to bond the materials by use of fastening members such as rivets.
These types of vacuum insulation panels are disclosed for example in Japanese Patent Publication Nos. 61-17263, 1-46759, and 3-23825.
The present invention aims at solving the problems related to using a vacuum insulation panel for a heat insulating travel container, which differs from the domestic refrigerator in the environment of use and the manufacturing method.
1) During transportation of the heat insulating travel container, the container is vibrated and deformed when traveling on a rough path or riding over a curbstone or the like, and this causes the wall to receive a bending or torsional load. In the wall structure of a domestic refrigerator, as explained, the vacuum insulation panel is adhered to the outer panel. When such a load is added to the wall, the stress will be directly transmitted to the vacuum insulation panel, and the intensity of the film may not bear such stress. As a result, the film may be torn. When the film is tom, the panel can no longer maintain a vacuum condition, and the heat insulating performance of the vacuum insulation panel is deteriorated.
Accordingly, when the vacuum insulation panel is utilized as a component of a wall for a heat insulating travel container, the vacuum insulation panel should be positioned near the center of thickness of the wall, so that when bending or torsional load is added to the wall, only a small stress is transmitted to the film of the vacuum insulation panel.
2) Generally, a forklift is utilized for loading the cargo in and out of the refrigerated travel container. At this time, there is fear that the claws of the forklift may bump into the insulating wall, or obstacles outside the container may hit the wall, causing damage to the outer panel.
If the vacuum insulation panel is positioned close to the outer or inner panels of the insulating wall, damage to the panels may cause the film to break, and the insulating performance of the vacuum insulation panel may be deteriorated.
Accordingly, when the vacuum insulation panel is used as a component forming the wall utilized in the heat insulating travel container, the vacuum insulation panel must be positioned in the center of thickness of the wall.
3) Moreover, various parts, rails and angles are fixed to the inner and outer panels of the container by fastening members (rivets). A pull-stem type rivet is normally used for the heat insulating container. Other rivets include a solid-type, a full-tubular type, a semi-tubular type, a split-type, a compression-type, a blind rivet and the like.
With reference to FIGS. 17 and 18, the method of fixing a pull-stem type rivet is explained.
A rib 16, a doorframe 17 and the like are mounted to the inner panel 130a and the outer panel 130b of the heat insulating travel container 15.
The method of fixing the rib 16 is explained.
A base hole 130c is formed in the inner panel 130a (or the outer panel 130b) with a drill. Thereafter, a rivet 18 is inserted into the hole, and the rivet is pulled and fixed by a riveter. This may cause no trouble to the sandwich panel, but if the vacuum insulation panel 25 is placed between the inner and outer panels 130a, 130b, and film 25b of the panel 25 may be damaged when drilling the base hole or when inserting the rivet into the hole.
Normally, a stopper is provided to the drill so as not to drill too deep, but the position of the stopper differs according to each manufacturer, and the depth of the base hole is not regular. For example, in this type of walls, the drill is provided with a stopper that stops the drill at a depth of 15 mm. In other words, the rivets could not be fixed to a base hole shorter than approximately 15 mm. Therefore, the vacuum insulation material should be placed in the center of thickness of the wall, with distances of 15 mm+a away from the inner and outer panels, respectively. The clearance size a should be set to approximately 10 mm, for safety when forming a base hole.
Other than the above members, parts that may be fixed to the heat insulating wall include the following: (The problems that are related to mounting these parts are the same as those for the rivet.)                a lashing rail (fixed to the inner panel) for fixing a lashing belt which prevents the cargo inside the container from moving due to vibration or the starting or the stopping of the vehicle;        a pallet guide (fixed to the inner panel) preventing the cargo inside the container from bumping into the side walls due to vibration or the starting or the stopping of the vehicle;        an air rib (fixed to the inner panel) for accelerating the dispersion of cool air within the container; and        an angle fixed to the interior and a rail fixed to the exterior of the corner portion, joining the walls together.        
(5) The conventional methods for determining the necessary distance between the vacuum insulation panel and the inner and outer panels, and the method of fixing the panel in position including the following:                1) Japanese Laid-Open Utility Model Application No. 4-68989 discloses placing a single-body vacuum insulation panel inside a flat panel-shaped mold and injecting a urethane foam material around the insulation panel, so as to manufacture a vacuum insulation panel unit with a urethane cover. The unit is placed between inner and outer panels. In this case, the vacuum insulation panel placed within the model for injection tended to move around the mold due to the expanding pressure of the urethane material, and it was very difficult to fix the vacuum panel in a determined position at the center of thickness of the wall material.        2) Japanese Patent Publication No. 2-9272 discloses a method of spraying a urethane foam on the inner panel or the outer panel, and while the urethane foam is gelling or expanding, adhering a vacuum insulation panel to the urethane material.        
According to the disclosed technique, there is a large dispersion in the state of expansion of the sprayed foam, and it is difficult to fix the vacuum panel in a determined position away from the inner panel or the outer panel.                3) Japanese Utility-Model Publication Nos. 1-20631 and 3-38628 disclose a wall formed by adhering a deforming sponge or plastic resin to inner and outer panels, and mounting a vacuum Insulation panel on that layer. A urethane foam material is injected around the vacuum panel, so as to cover the outside of the panel. However, since the deforming sponge and deforming plastic resin are deformed by the expanding pressure of the urethane foam, it is difficult for the vacuum panel to be fixed in a determined position away from the inner and outer panels.        4) Japanese Laid-Open Patent Applications Nos. 3-233285, 8-14484 and 8-14486 disclose fixing a vacuum insulation panel in a desired position in the width of a wall by a fixing jig. However, since the fixing jig itself has a very high heat conductivity, a heat-bridge is generated within the wall, and it is difficult for the wall to provide a sufficient heat insulating performance.        